ICL7135 to Printer port Interface - del20002

A PC based Analog Voltmeter. The Printer Port or
Parallel Port Interface is used to acquire high
resolution analog data using ICL7135. This is a
method of getting analog data into your PC. This is
only of Educational Value. Applications include Data
Loggers and PC based Process Monitors, chart
recorders. (USB, Wireless Interfaces and Tablet
Computers are in vogue today)

The ICL7135 details can be seen here 41/2
A/D with BCD ICL7135 . This Mixed device was
far ahead of its times, a product of Intersil,

The four Digit Drive outputs and four BCD outputs of
the Multiplexed Display Driver of 7135, along with
A-D status output which is also four, are routed to
four input pins on printer port via three 74HCT373
Digital Switches. This is because we have less Input
pins on Port and 4 are shared by making one 74HCT373
transparent and others High Impedance when PC takes
a reading. So the three sets of four outputs of 7135
can be read sequentially.

To
view circuit click the Link of PNG or PDF, The PNG
Image can be Drag Scrolled with Mouse.

74HCT139 U9A which is controlled by the PC program
can select U3, U4 or U5 for sending data to 4 pins
on printer port. Data sent on the eight output
pins of printer port can be latched by U10B
into U6. This eight outputs of U6 can drive relays
or lamps.

The Design source files in Orcad Capture 9 format is
here 20002.zip. A
example VB program with source to test above circuit
with PC is here, Visual Basic
Digital Voltmeter. This program may not be
error free but works.

When you build above circuit and voltmeter on PC.
Use it only with 5V battery projects. Any high
voltage goes to printer port due to wrong connection
you may lose a motherboard or worse. Better
isolate all data lines to port with opto couplers if
you want to build this for regular use